The present invention relates to a process for producing citric acid soluble potassium silicate fertilizer in a high yield and with a high heat efficiency, and to an apparatus for practicing said process.
As is well known, a citric acid soluble potassium fertilizer can be solubilized by the citric acid excreted by the roots of plants, but only a certain amount of the fertilizer is solubilized and absorbed into the plant. Unlike the usual water-soluble potassium fertilizers, the citric acid soluble potassium fertilizer is resistant to rain water. Further, it neutralizes acid in soil caused by the citric acid excreted by plant roots. Although such a citric acid soluble potassium fertilizer is suitable for Japanese agriculture where the climate is relatively warm and rainy and the acidification of soil is already at an advanced stage due to the use of chemical fertilizers, such as ammonium sulfate, its practical use is only rare. This is mainly attributable to the fact that if a citric acid soluble potassium silicate fertilizer is produced by the conventional process from fly ash as a silicic acid source, a continuous operation is quite difficult to conduct because of the high temperature necessary for calcination, which inevitably results in high cost as compared with other fertilizers. In addition, such fertilizers produced using known techniques lack uniform product quality.
Earnest studies have been conducted with the aim of overcoming the above-mentioned difficulties encountered with citric acid soluble potassium fertilizers. As a result, it has been discovered that an inexpensive potassium fertilizer of high quality can be produced with a high efficienty by utilizing fly ash, such as that formed in power stations operated with coal, together with inexpensive crude caustic potash or alcoholic waste liquor.
Fly ash is contained in waste gases formed in boilers and furnaces operated with pulverized coal, such as those in thermal power stations. The fly ash contains a large quantity of silicon. If the fly ash is admixed with a potassium source, such as caustic potash (KOH) or potassium carbonate and calcined at a temperature of about 600.degree. C. to 1,100.degree. C., the potassium reacts with the silicon present in fly ash to form a citric acid soluble potassium silicate. The chemical reactions that occur can be expressed by the following chemical equations: ##EQU1##
As is well known, fly ash has a size of micron order, so that if one wishes to calcine it, such as by means of a fluid calcining furnace, it scatters in the hot air used as the calcination medium, and the calcination cannot be carried out.